mm/pagewalk: fix EFI_PGT_DUMP of espfix area

Booting x86_64 with CONFIG_EFI_PGT_DUMP=y shows messages of the form
"mm/pgtable-generic.c:53: bad pmd (____ptrval____)(8000000100077061)".

EFI_PGT_DUMP dumps all of efi_mm, including the espfix area, which is set
up with pmd entries which fit the pmd_bad() check: so 0d940a9b warns
and clears those entries, which would ruin running Win16 binaries.

The failing pte_offset_map() stopped such a kernel from even booting,
until a few commits later be872f83 changed the pagewalk to tolerate
that: but it needs to be even more careful, to not spoil those entries.

I might have preferred to change init_espfix_ap() not to use "bad" pmd
entries; or to leave them out of the efi_mm dump.  But there is great
value in staying away from there, and a pagewalk check of address against
TASK_SIZE may protect from other such aberrations too.

Link: https://lkml.kernel.org/r/22bca736-4cab-9ee5-6a52-73a3b2bbe865@google.com
Closes: https://lore.kernel.org/linux-mm/CABXGCsN3JqXckWO=V7p=FhPU1tK03RE1w9UE6xL5Y86SMk209w@mail.gmail.com/
Fixes: 0d940a9b ("mm/pgtable: allow pte_offset_map[_lock]() to fail")
Fixes: be872f83

 ("mm/pagewalk: walk_pte_range() allow for pte_offset_map()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>